The present invention relates to monitoring the condition of field devices. Specifically, the invention relates to a system for detecting the presence of fluids in the field devices.
In many industrial settings, control systems are used to monitor and control inventories, processes, and the like. A typical control system includes a centralized control room and a number of field devices geographically removed from the control room. The field devices communicate process data to the control room using either analog or digital communication means.
Traditionally, analog field devices have been connected to the control room by two-wire twisted-pair current loops, with each field device connected to the control room by a single two-wire twisted pair loop. Located within the field device housing are terminals for connecting the twisted-pair current loops to circuitry within the field device. This region is referred to as the terminal block area of the field device. Typically, a voltage differential is maintained between the two wires of approximately 20 to 25 volts, and a current between 4 and 20 milliamps (mA) runs through the loop. An analog field device transmits a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. A receiving device measures the voltage across a load resistor, typically located in the control room, in order to determine the magnitude of the modulated current.
While historically field devices were capable of performing only one function, recently, hybrid systems that superimpose digital data on the current loop have been used in distributed control systems. The Highway Addressable Remote Transducer (HART) and the Instrument Society of America (ISA) Fieldbus SP50 standards superimpose a digital carrier signal on the current loop signal. The HART standard employs frequency-shift keying (FSK) to transmit digital data over the current loop, and operates at frequencies of 1200 and 2400 baud. Other common protocols for communication of digital information over the current loop are Foundation Fieldbus, Profibus, and DeviceNet. Typically, these systems operate at much higher frequencies than the HART protocol. The digital carrier signal can be used to send secondary and diagnostic information. Examples of information provided over the carrier signal include secondary process variables, diagnostic information (such as sensor diagnostics, device diagnostics, wiring diagnostics, process diagnostics, and the like), operating temperatures, sensor temperature, calibration data, device ID numbers, configuration information, and so on. Accordingly, a single field device may have a variety of input and output variables and may implement a variety of functions.
Field devices are often located in physically challenging environments, with one potential problem being the collection of fluid within the terminal block area of the field device. The presence of fluid within the terminal block area can result in a conductive path between the terminals of the field device, known as terminal leakage. The presence of terminal leakage in a field device affects the ability of the field device to accurately communicate the measured process variables to the control room. The ability to accurately communicate the values associated with a measured process variable to the control room is important to ensuring proper operation of control processes. It is difficult and time-consuming, however, to periodically inspect each field device for the presence of fluid in the terminal block. Therefore, it would be beneficial to design a system for automatically detecting the presence of fluids with the terminal block area of field devices.